Chromosome segregation is a complex process that requires the precisely coordinated action of many proteins. The interactions and functions of these proteins can be elucidated genetically by examining the consequences of their malfunction in mutants. This information is important both because it helps elucidate normal function and because it leads to better understanding of the errors that can occur. Errors in chromosome segregation cause a variety of genetic disorders, including Downs Syndrome. The proposed research is designed to investigate the molecular basis of chromosome segregation in yeast. It makes use of mutations in genes that have essential functions during chromosome segregation, including DNA topoisomerase II. It also includes novel method of producing such mutations. This project has four goals: 1) To clarify the role of DNA topoisomerase II in the highly specialized first division of meiosis, using temperature-shift experiments and double-mutant analysis. 2) To investigate the role of topoisomerase II in chromosome structure, by genetically identifying the proteins with which it interacts and analyzing their function. 3) To analyze the molecular function of cdc gene products specifically involved in chromosome segregation, by cloning the genes by complementation, sequencing them, making new mutations by in vitro mutagenesis, and using antibodies to determine the cellular locations of the gene products. 4) To develop a novel method for identifying new mutations affecting gene products that are important for proper chromosome segregation.